Add dynamic splitting to JdbcIO.readWithPartitions

[damccorm]

Now, the JDBC IO is basically a DoFn executed with a {}ParDo{}. So, it means that parallelism is "limited" and executed on one executor. ReadWithPartitions does some preliminary partitioning of the data, but any skew in data range or workload will create an unbalanced workload.

 

Imported from Jira BEAM-14161. Original Jira may contain additional context.
Reported by: pabloem.